1. Field of the Invention
The invention relates to a window antenna for a motor vehicle having a thin electrically conductive layer which is transparent and reduces heat transmission.
2. The Prior Art
Antennas of this type are known from German Patent Application No. P 197 35 395. If disposed in the front windshield they suffer from interference due to digitally operating vehicle equipment. In the case of a front engine, interference results from the ignition system. For this reason, rear window antennas have been used in the past. The heating elements of the window heater were used containing a conductive layer which reduces heat transmission. To prevent the unfavorable impedance conditions due to the supply of heating current, the heating current for rear window antennas must always be supplied through an RF choke circuit. This choke circuit is particularly complex, especially for frequencies in the LMS region. For this reason, flat antenna conductors are used to receive LMS signals, which in many cases are offset from the heating surface, as are known from U.S. Pat. No. 4,791,426. In the case of a continuous conductive coating however, the antenna cannot be used without additional measures.
An object of the present invention is to provide an antenna having good reception characteristics both in the USW and TV region and in the LMS region while minimizing its complexity.
The antenna according to the invention provides special advantages compared with known antennas of the type which are based on heating areas with complex RF choke circuits in the region of long, medium, and short wave reception (LMS frequency region). By virtue of the long wavelength, the operating principle of an antenna according to the invention can be described by its capacitive effects, while the inductive effects can be disregarded.